German Published Patent Appln. No. DE 41 32 851 A1 discloses a bar coil as the ignition coil unit, having a cylindrical casing in its basic form. A rod-shaped core, a primary winding and a secondary winding, each applied to a separate bobbin, are inside the casing, together with several return plates to influence the magnetic field of the bar coil. These elements are separated from one another by gaps, some of which have a small cross-sectional area and must be filled with casting resin without defects to insulate the elements. The casing is closed at one end with a primary terminal and at the other end with a secondary terminal as the high-voltage terminal, so that it is difficult to add the casting resin.
The narrow gap results in an unfavorable filling time from the manufacturing point of view, and the function is endangered by high-voltage sparkovers due to the possible development of air inclusions.